Push-pull current source



June 30, 1970 D. C. HERRMANN ET AL- PUSH-PULL CURRENT SOURCE Filed March 50, 1967 INVENTOR! Jan/a0 8%!0/41/1/ a z #04; JJ4z/4/ mun United States Patent 3,518,457 PUSH-PULL CURRENT SOURCE Donald C. Herrmann, Hartsville, Pa., and Lucas J. Bazin,

Stratford, N.J., assignors to RCA Corporation, a corporation of Delaware Filed Mar. 30, 1967, Ser. No. 627,110 Int. Cl. H02j 1/04 U.S. Cl. 307-297 4 Claims ABSTRACT OF THE DISCLOSURE Two opposite conductivity transistors are serially connected together for current flow between opposite polarity sources of voltage with their collector electrodes connected together and forming a high impedance source of constant current for a load. In series circuit with the transistors are oppositely variable resistors to adjust the magnitude and polarity of the current supplied to the load.

It often is necessary to provide facilities for maintaining a substantially constant current flow in a load circuit irrespective of changes of the impedance of the load which might be caused by temperature variations, for example. One such instance is in the operation of certain television camera tubes, particularly of the photoconductive type, such as the vidicon and isocon. Each of these tubes is provided with one or more coils in which the current must be adjusted to a particular value and that value of current maintained in order for these tubes to operate successfully. Both the vidicon and isocon have alignment coils by which to properly position the electron beam relative to the target electrode of the tube to be scanned by the beam. In addition, isocon tubes require so-called steering coils by which to effect the proper landing upon the electron multiplier dynode electrode of the return electron beam from the target electrode. It is necessary for proper operation of tubes of this character that the current in the coils be not only adjustable to achieve the desired operation of the tubes, but also that the current be maintained substantially constant during the period of operation. Such apparatus is used in environments which are subject to temperature changes which result in changes in the resistance of the coils and certain of the circuit components supplying the coil current. Such resistance changes tend to vary the current in the coils.

It is possible to provide a substantially constant source of current for such coils by employing current supply circuits having high resistance components, such as resistors and otentiometers but it is necessary to employ relatively high voltages and therefore high power across such a circuit in order for it to operate satisfactorily. There is need, therefore, for a substantially constant current source without having to resort to the use of such components and the attendant high voltages.

It is an object of the present invention to provide a source of substantially constant current which also, preferably, is adjustable for both current magnitude and polarity, and utilizing only relatively low voltage supplies, with low power.

In accordance with an illustrative embodiment of the invention, two relatively low voltage sources of opposite polarity relative to a reference potential are connected across a series circuit including resistive elements, two of which are oppositely variable. The series circuit also includes emitter-collector electrode circuits arranged for current flow from the voltage sources. The collector electrodes of the transistors are connected together and constitute a relatively high impedance source of current which is connected to the load circuit. The magnitude and "ice polarity of the current supplied to the load is varied by adjustment of the variable resistors. The base-emitter electrode circuits of the transistors are permanently connected to the respective voltage sources by means including diodes in a manner to maintain substantially constant the emitter-collector current flow through the respective transistors irrespective of any changes which may occur in the supply voltages.

For a more detailed disclosure of the invention, reference may be had to the following description which is given with reference to the accompanying drawing of which:

The single figure is a schematic circuit diagram of an illustrative embodiment of the invention.

In the drawing, an PNP transistor 11 is connected in series with an NPN transistor 12 with their respective collector electrodes connected together and to a load circuit represented by a coil 13. The emitter electrode of the transistor 11 is connected by means of a fixed resistor 14 and a variable resistor 15 to a positive voltage source 16, Similarly, the emitter electrode of the transistor 12 is connected by a fixed resistor 17 and a variable resistor 18 to a negative voltage source 19. As may be seen, the voltage sources 16 and 19 nominally have the same voltage but opposite polarity. Also, the fixed resistors 14 and 17 and the variable resistors 15 and 18 have the same respective nominal values. The adjustable contacts of resistors 15 and 18 are mechanically interconnected so that as one resistor increases in value, the value of the other resistor decreases in like magnitude.

The respective base electrodes of the transistors 11 and 12 are connected to the voltage sources 16 and 19 by diodes 21-22 and 23-24. The base electrodes of the transistors also are connected to opposite ends of a resistor 25. Current flow from the voltage sources 16 and 19 through the resistor 25 and the diodes 21, 22, 23 and 24 produces suitable voltages at the base electrodes of the respective transistors 11 and 12 to effect proper operation of the transistors.

The magnitude and polarity of the current through the load coil 13 is controlled by concurrently adjusting the variable resistors 15 and 1-8. This has the effect of varying not only the polarity of the voltage to the output point 26, but also its magnitude relative to a reference potential such as ground, for example. The impedance of the current source at the output point 26 is high by reason of the fact that the output current is derived from the collector electrodes ofthe transistors 11 and 12. In the illustrated circuit, this impedance is in the range of to kilohms. Consequently, if by reason of temperature or other changes, the resistance of the coil 13 changes during operation, the current supplied to the coil is maintained substantially constant because of the high impedance character of the source as represented at the output point 26.

The current to the load coil 13 also is made independent of any changes in the supply voltage sources 16 and 19 by the inclusion of the Zener diodes 21 and 24 in the base-emitter circuits of the transistors 11 and 12. Any change in one or the other of the sources of voltage effects the same change to both the emitter and base electrodes of each of the transistors, thereby maintaining a constant emitter-collector current in the transistors at any particular adjustment of the variable resistors 15 and 18. The function of the diodes 22 and 23 in the base-emitter circuits of the transistors is to balance, or offset, the relatively small base-to-emitter voltage of the transistors.

It may be seen from the component values given in the figure that there is provided in the present invention a substantially constant source of current for a load which does not require components of large value and/ or voltage sources of relatively high magnitude. It has been found that such a circuit operates satisfactorily even though the load represented by the coil 13 and/or components traversed by the coil current may be subjected to conditions such as temperature changes which produce changes in the resistance in the load circuit. In fact, the consistency or stability of the load current is limited only by the stability of the resistive components 14, 15, 17 an d 18 in the emitter circuits of the transistors 11 land 12. What is claimed is:

1. A source of constant current for a load, comprising: a pair of opposite conductivity transistors each having base, emitter and collector electrodes; two voltage sources of opposite polarity with respect to a common reference potential; I first means coupling the collector electrodes of said transistors together; second means coupling said load to said collecter electrodes; third means coupling the base electrodes of said transistors to said tWo voltage sources; and fourth means including resistors coupling the emitter electrodes of said transistors to respective ones of said voltage sources, said resistors being adjustable and linked together to increase the resistance of one with a corresponding decrease in resistance of the other for determining the magnitude and polarity of current in said load. 2. A source of constant current for a load comprising: a pair of opposite conductivity transistors each having base, emitter and collector electrodes; two voltage sources of opposite polarity with respect to a common reference potential; first means coupling the collector electrodes of said transistors together and the emitter electrodes of said transistors to respective ones of said voltage sources; second means coupling said load to said collector electrodes for providing a relatively high impedance source of substantially constant current for said load;

third means coupling the base electrodes of said transistors to said two voltage sources in a manner to cause a current flow in the emitter-collector circuits of said transistors;

said first coupling means including resistors connected in said emitter electrode circuits to said respective voltage sources, the relative values of said resistors determining the magnitude and polarity of current in said load, one of said resistors in each of said emitter circuits is adjustable; and

means linking said adjustable resistors to increase the value of one of said adjustable resistors and to concurrently decrease the value of the other of said adjustable resistors by a like amount.

3. A source of constant current for a load as defined in claim 2, wherein:

said third coupling means includes a Zener diode coupled between each of said transistor base electrodes and its associated voltage source to maintain a substantially constant voltage difference between the base and emitter electrodes of said transistor irrespective of any changes in said voltage sources.

4. A source of constant current for a load as defined in claim 3, wherein:

said third coupling means includes a diode coupled between each of said transistor base electrodes and its associated voltage source to provide a voltage drop to offset the base-to-emitter voltage drop of its associated transistor.

References Cited UNITED STATES PATENTS 3,263,093 7/1966 Erdmann 307-228 3,363,191 1/1968 Boughtwood et a1. 330-13 3,402,353 9/1968 Hubbs 307-263 3,114,872 12/1963 Allard 307296 DONALD D. FORRER, Primary Examiner D. M. CARTER, Assistant Examiner US. Cl. X.R. 

